1. Field of the Invention
This invention is in the field of local area networks in which a plurality of physical modules of the network communicate with one another over a network bus, and more particularly relates to methods and apparatus of synchronizing the timing subsystems of each of the modules so that they are synchronized within a predetermined degree of accuracy.
2. Description of the Prior Art
A computerized plant management system is described and claimed in U.S. Pat. No. 4,607,256, entitled PLANT MANAGEMENT SYSTEM by Russell A. Henzel, which application is assigned to Honeywell Inc., the assignee of this application. The disclosure of Application No. 06/540,061 is hereby incorporated by reference into this application. Such a system is composed of a plurality of physical modules having varying capabilities and functionalities which communicate with one another over a common communication medium, or local control bus, to form a token-passing local area network. Each of the physical modules of the network is the equal, or peer, of the other, and each of the modules includes at least a module central processor unit (MCPU), and a module memory unit (MMU). Additional controllers and devices are added to a physical module to provide it with the ability to perform desired functions. A network of this type provides a distributed data processing environment with a concomitant increase in reliability over centralized systems since if one module fails, the network as a whole is not disabled as would be the case with a failure of a centralized system. Reliability is also improved by permitting redundancy of the physical modules of the network to the extent necessary to achieve desired system availability. Such a token-passing local control network consisting of a plurality of different types of physical modules also permits functional capabilities to be added or deleted incrementally.
One of the requirements for a computerized plant management system is that of timing the occurrence of events with a high degree of precision. A centralized timing system which could be used to satisfy the timing requirements of such a plant management system would do so to the detriment of the systems' objectives of improved reliability through redundancy at the module level, of minimizing the cost of the system, and of providing additional capabilities, or modifications, to the network through the addition and deletion of physical modules since a centralized timing subsystem could not readily satisfy these objectives.